Major automakers are scrambling to strip hundreds of pounds off future pickup trucks to meet new U.S. standards for fuel economy without sacrificing strength or towing capability.

The new mandates take effect in 2016, giving automakers such as Ford and General Motors just one design cycle to make significant changes that will require costly steel substitutes including aluminum, new steel alloys and magnesium.

Automakers are faced with having to pass on those higher costs to consumers who have come to associate mass with performance.

"There is a lot of hand-wringing in the industry right now," said Dick Schultz, managing director of the Automotive Materials Practice at Ducker Worldwide and expert in the use of metals in autos. "You can't afford to be on the wrong side of this thing."

Automakers must reach an average fleet fuel economy of 35.5 mpg by 2016.

Light trucks — which were half of all U.S. auto sales in the first 11 months of 2010 — will have to get about 30 mpg.

The corporate average fuel economy (CAFE) standard for 2010 is 29.2 mpg. For light trucks alone, it is 24.9 mpg, according to government data.

The updated standards come at a time when auto companies are launching an array of battery-electric, plug-in and hybrid vehicles, which will help the sector reach those new goals.

But reducing the weight of their trucks is also critical to meeting the new guidelines, automakers say.

This represents a significant challenge because of the trucks' large size and the demand that they be able to handle heavy loads and towing in unforgiving conditions.

Current pickups weigh an average of nearly 5,000 pounds.

Automakers have added comfort, electronic and safety features over the past decade. As a result, the weight of trucks has jumped 22 percent from 2000 to 2010, federal data show, while fuel economy rose only 2 percent.

The first automaker out with a new-model large pickup truck aiming to comply with the tougher fuel economy standards will be GM with its Chevrolet Silverado for the 2014 model year.

"It's a tough task, but we're facing it as grown-ups," said Rick Spina, who leads full-size truck development for GM. "We're going to do everything we can to keep the customer from realizing we've had to make changes in a fundamental way."

In addition to the 2016 target, automakers may have to achieve CAFE standards of 62 mpg for the overall fleet by 2025, under the most ambitious scenario outlined by the U.S. government.

GM aims to shed 500 pounds from its trucks by 2016, and by the early 2020s might need to cut as much as 1,000 pounds per truck, Spina said.

Using blown-in foam instead of a cheaper, but heavier, pad to buffer noise in certain areas of the vehicle could become more common, he said.

Meanwhile, Ford is looking closely at a magnesium-alloy frame for the next generation of its F-150 pickup truck, two people familiar with the matter said. Ford is also looking to use aluminum for the body panels of the F-150, they said.

By moving away from traditional steel, Ford could shave about 800 pounds off the truck, one person said.

The comments were made privately because the information is not yet public.

Ford declined to comment on its specific product plans.

GM is also exploring the use of aluminum and magnesium for the frames of future models of its Chevrolet Silverado and GMC Sierra trucks, Spina said.

Ford has been aiming to trim as much as 700 pounds from its vehicles under a target it has made public, Ford's global product development chief Derrick Kuzak said. Ford's F-Series is the No. 1 selling vehicle in the United States, followed by GM's Silverado.

Automakers are starting to tout fuel economy as part of the brawny image of trucks.

"You won't be put in a chokehold every time you fill up because you can also get the best fuel economy," actor Denis Leary says in a TV ad for Ford trucks.

But past advances in vehicle engineering, including the use of lighter materials, have proved hard to sell to consumers.

Ford, for example, abandoned an experiment with the heavy use of aluminum in a limited number of Mercury Sables built in the mid-1990s.

The experiment cut more than 350 pounds but would have added hundreds of dollars in costs, Schultz said.

Eric Fedewa, IHS Automotive director of global powertrain forecasting, said the additional costs could squeeze the truck market as a share of overall sales.

"With fuel economy standards where they are, trucks are going to get kind of edged out of the top of the market," Fedewa said. "Everything is going to change in the next vehicle cycles."